Finishing a 400 Amp service started by another contractor. A 3/4 EMT was run from the MDP to the building steel for a 1/0 ground. I was told a long time back not to do this, that the metal conduit could "choke" the ability of the ground to dump a large fault. Can't find any reference to this in NEC. Any truth to what I was told or is running in the EMT practical?

This is from 250-64. (It's a cut/paste, sorry if the spacing screws up.)

"(B) Securing and Protection from Physical Damage. A grounding electrode conductor or its enclosure shall be securely fastened to the surface on which it is carried. A 4 AWG copper or aluminum or larger conductor shall be protected if exposed to severe physical damage. A 6 AWG grounding conductor that is free from exposure to physical damage shall be permitted to be run along the surface of the building construction without metal covering or protection where it is securely fastened to the construction; otherwise, it shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor. Grounding conductors smaller than 6 AWG shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor."

This is from 250-92"(3) Any metallic raceway or armor enclosing a grounding electrode conductor as specified in 250.64(B). Bonding shall apply at each end and to all intervening raceways, boxes, and enclosures between the service equipment and the grounding electrode."

Poorboy,You are VERY correct, it totally negates a grounds ability to clear a fault in a timely manner. This is perhaps the most heinous of mistakes made by your average electrician and especially, lightning protection contractors.

Mr. Miller has provided the answer though, in that if you bond both ends, you're back to having a ground.

Thanks for your thought and follow through on the design and implementation of your job, we need lots more of this in the field.

Chimo, this is the classic difference between electric and electronic thinking. A ground wire in a single end grounded metal raceway will certainly clear the fault ... eventually. The problem is when the O/C device will actually see the leading edge of the fault. The choke effect tends to delay the edge, allowing things to burn a little longer. This really becomes important when you are trying to protect electronics. Metal raceways should be bonded at both ends for optimal protection. It has been in the code for metal enclosed ground electrode conductors as long as we can remember and that is the function of the skinny bond wire in AC cable.

A conductor inside a conduit which is only grounded at one end forms a "delay line", which consists of the distributed inductance of the wire and the conduit, and the distributed capacitance between them.

If one end of the conductor is hit with a very fast risetime pulse (like a lightning surge), the current takes a certain amount of time to charge the inductance and capacitance of the delay line, during which time the voltage can build to dangerous levels at the ungrounded end.

Does this really have anything to do with clearing a fault (operating an OC device) faster?

Lightning produces fast rise-time events on the order of a few microseconds, and the delay-line effect is in the same range. How can these affect the response time of an overcurrent device, which operates in a range of tens to hundreds of milliseconds?

I'll accept the explanation that it's to reduce the effective inductance of the wire, improving its ability to conduct fast-transient surges. After all, a wire running through a ferrous tube is topologically equivalent to a single turn through a toroidal core (a skinny, but very, very thick core).

None of the grounding had been done when we contracted to finish this service. We ran a 1/0 bare copper 15 feet to the 2" copper water main and straight thru the lug to the iron sprinkler main, a #6 to the 2 ground rods just outside, and another 1/0 thru the 3/4 EMT(10 foot length) to the building steel. The EMT exits the panel thru an EMT connector and the 1/0 protrudes out the top of it for 5 feet before reaching the lug.

If it seems worth it I can simply loosen up the 1/0, slide a connector on to the EMT and add a grounding bushing. Are you saying a piece of #6 out of the panel running up adjacent to(not inside of) the EMT and connected to the grd bushing would make this a better install?

BTW, Gfretwell's comment on the skinny ground in the AC cable was very interesting to me, never knew that.

Sorry for the hesitancy in an answer to the question. The subject is long, difficult and complex. In Other Words, it requires painful thought – I HATE when that happens. There is the temptation to use the old standby “Because it’s in the code”. In the case of the 1999 NEC (Because that’s what Virginia uses), it is in 250-92 (a)(3).

In order to understand the principle we must remember, and understand, the basic concept of the ground. Basically stated:

The concept of the ground is to force (give the current clear path) fault current to a high enough level to clear the fault, or open the OC device in a timely fashion. This is most often defined as 5 cycles.

The explanation of that principle normally takes me 6 – 12 hours in class. Let’s face it, Bill would bar me from the board for that. If you do not accept that as the basic principle of grounding, then no explanation can satisfy the question.

Do you accept the concept of transformers? That power applied t o the primary winding “magically” makes power appear on the secondary winding. There is no connection between the windings, but merely by inductive effect the secondary is drawing (using, wasting) power from the primary, regardless if anything is connected to it.

Transformers have a high efficiency because of laminated iron/steel cores, winding size and placement, insulating material, etc.

How efficient is this principle when it is a conductor (bare or insulated) laying in a metallic cover? In this case – we have an energized conductor (during a fault) and by the same inductive effect it has energized it’s metallic covering during a period we want the current at it’s highest level to open the OC device. Is it attached to a conductive surface, such as a steel bar joist or column/beam, or even concrete? Then we’ve energized that portion as well again robbing us from our fault clearing power.

The conduit bonded together at each end to the Grounding Electrode Conductor is a clear parallel path, not bonded at either end it has the same sympathetic relationship that transformer windings have, bond it at only one end and we have a series circuit with sympathetic values at the unbonded end. The series circuit configuration will not only draw power from your critical fault path but it will arc internally and likely throw some very hot sparks on the path – every time you have a fault, large or small. A simple fault on a branch circuit upstream places a current value back here as well.

What are the sparks? You’re losing conductor and/or conduit every time you fault or current goes high, how does this effect your future performance? Let’s say it does not improve your values, or performance.

The 7th edition of “Soares Book on Grounding”, on page 107, Chapter 7, finishes their explanation this way “Where that bonding procedure is not followed, the impedance of the grounding electrode conductor is approximately doubled with the result that it’s effectiveness is markedly reduced”.

That’s about it in a nutshell, hope it helps. As always, I strongly suggest “Soares Book on Grounding” for a deeper explanation, with pictures.

BTW, 1959 NEC, 250-71 requires the same thing, says it a little bit differently, but actually makes it more clear. This is NOT a new requirement.

Like I said earlier, “Because the Code says so” and has for a LONG time.